Figure out how to handle string differences between mullvad release and mullvad tip

In tor-browser, we merge the strings used in the release/stable branch and the tip using our CI, before sending the file to weblate. And then pulling them in for the build. See tor-browser#42305 (closed).

The main reason to do this is to ensure that strings can be removed from the development tip, whilst remaining available in the release builds. Mullvad should have something similar in place, but I'm not sure about mullvad's translation process.

/cc @ruihildt @pierov could you fill me in on all the details about how a translation file in the mullvad-browser repository makes its way to translators, and all the way back to the mullvad browser build.